Spreading and bouncing of liquid alkane droplets upon impacting on a heated surface

Abstract

This paper reports an experimental investigation on the impact dynamics of liquid normal alkane (n-heptane, n-decane and n-tetradecane) droplets on a stainless steel surface using high speed photography and long distance microscopic techniques. Particular interest is paid to comprehensively explore the effects of liquid viscosity and surface roughness on droplet spreading and bouncing dynamics at different thermal hydrodynamic impact regions. Specifically, firstly, high speed images identified four regimes (evaporation, nucleate boiling, transition boiling and film boiling regime) of physical phenomena that couple the droplet spreading hydrodynamics, heat transfer and phase change. Bubbles generation due to the heating of the surface with compression of air disk under the droplet was observed and this phenomenon is firstly promoted and then inhibited with the increase of the wall temperature until finally no bubbles were observed when wall temperature is beyond the Leidenfrost point (TL). Rim disturbances during spreading were observed at relatively high Weber number with wall temperature higher than TL. Increasing wall temperature reduces the rim disturbance. Secondly, the measured non-dimensional maximum spreading diameter βmax decreases with the increase of surface temperature until it becomes a constant when temperature is beyond TL. Rough surface was found to have a lower TL because of larger vapor pressure provided by more nucleation sites. Finally, for wall temperature beyond TL, droplet bounces up after a certain period of residence time (τr). It takes more time for droplet to rebound at larger We because of larger βmax takes longer time to retract and rebound. Both surface roughness and liquid viscosity showed no influence on time to reach βmax (τmax), but significantly increases τr by slowing the retracting process, which both should be considered in future model of τr.